Foundations embedded in submerged soil

Abstract

A subsea foundation exemplified by a monopile or a suction pile comprises an embeddable body that extends along an upright embedment axis. A plurality of fins extend outwardly from the body in directions transverse to the embedment axis. At least some fins of the plurality are at least partially embedded in seabed soil. The plurality of fins is divided into an upper array of upper fins and a lower array of lower fins. The arrays may be spaced apart or overlap along the embedment axis, and may be angularly aligned or angularly offset with respect to each other about that axis.

Description

This invention relates to foundations that are embedded in submerged soil. The invention relates especially to the problem of scour due to erosion of the soil, and the consequential need to protect foundations of structures that rely upon submerged soil for support. The invention has particular benefit for protecting such foundations in open sea locations, such as monopiles or suction piles that may be used as foundations for bottom-fixed offshore wind turbines, and for maintaining the load-bearing capacity of such foundations. Scour can occur where water flows around an object that stands above the bed of a body of water in marine, riverine and lacustrine environments. Examples of such objects include upright elongate structural elements such as bridge piles, suction piles, legs of offshore oil platforms and monopile foundations of offshore wind turbines. A lower end of such an object is typically embedded in the soil, in which case the stability of the object often relies upon the depth of its embedment into the soil and the uniformity of embedment around the periphery of the object. In open seas, water flows could come from any direction. On encountering and dividing around a submerged stationary object, the divided water flow tends to generate eddies or vortices around and downstream of the object. Such vortices may entrain, transport and erode the soil in a process of excavation that, over time, decreases the embedded depth of the object and so could undermine its bearing capacity and stability. The problem of scour is especially acute around a monopile or a suction pile. Typically, such piles have a generally smooth cylindrical surface of circular outer cross-section. As is well known, a cylindrical body presents challenges of vortex formation when exposed to fluid flowing in a direction transverse to the central longitudinal axis of the cylinder. Additionally, where the seabed intersects the cylindrical surface, a ‘horseshoe vortex’ can develop close to the interface between the monopile and the seabed. A horseshoe vortex follows an arc that wraps around the upstream side and approaches the downstream side of the monopile. In that vortex, water tumbles about a curved horizontal axis in a manner that strongly promotes scour. Various approaches have been proposed to mitigate scour around objects underwater such as the foot or pile of a fixed offshore wind turbine. A first family of solutions involves shrouding, stabilising or armouring seabed soil by placing mats, rocks or other items upon the seabed around the turbine. For example, KR 10-0652171 discloses geotextile bags, GB 2529252 discloses mattresses of interlinked used vehicle tyres, and EP 3736382 discloses scour prevention units that each comprise a bag body housing block objects in a bag material. Other similar solutions involve placing frond mats or sediment-collecting mesh onto the soil. All of the above solutions are time-consuming and expensive to install and for many of them, efficiency or practicality is not fully proven. For example, rock armour solutions could require up to 5000 tonnes of rock per installed foundation. Obtaining such large quantities of rock can involve prohibitive cost, both monetarily and environmentally, and may not be practicable in some parts of the world. Also, the under-pressure of a horseshoe vortex tends to draw seabed sediments up through a porous armour layer such as rock armour, causing the armour layer to sink into the seabed over time. A second family of solutions involves placing geometrically-shaped anti-scour structures near to or around the foot of a wind turbine to impede vortex formation or to disrupt vortices once formed. For example, CN 216156628 and CN 113186986 teach various arrangements of blades, spoilers, wings or fins that are mounted to or in the vicinity of a monopile, at or above the seabed. CN 217267593 discloses vertical triangular plates that are spaced angularly around a monopile in respective radial planes extending from a sleeve that is concentric with the monopile. The circumferential gaps or spaces between successive plates are filled with sand or gravel and are then covered by respective curtains in an umbrella-like arrangement that is intended to combat scour by modifying water flow. It is also known to embed fins of a monopile or suction pile into the seabed to increase the surface area of contact with the seabed soil, hence increasing the load-bearing capacity of the pile but without otherwise enlarging the tubular wall or skirt of the pile. For example, CN 202265837U discloses fully embedded fins, as does OTC-30973-MS Effect of Fins on Combined Loaded Suction Caisson in Deepwater Clay Soils presented at the Offshore Technology Conference in Houston, Texas on 16 to 19 August 2021. Conversely, CN 110984214 discloses partially embedded fins where the top of each fin emerges from the seabed to provide a compartment for rockfill intended to stabilise the seabed and to protect the pile against scour. CN 213897201 and CN 219653750 disclose arrangements in which a structure that serves to protect and stabilise a pile foundation is received on the pile as a sleeve that engages the seabed. Similarly, US 2021 / 0348597 proposes a pile foundation including a floor slab that engages the seabed, the floor slab acting to increase resistance of the foundation to horizontal loads. Against this background, the invention resides in a subsea foundation that comprises a body extending along an upright embedment axis and a plurality of fins that extend from the body in directions transverse to the embedment axis. The plurality of fins is divided into an upper array of upper fins and a lower array of lower fins. At least some fins of the plurality can be at least partially embedded in seabed soil. The fins may lie in respective radial planes that converge on and contain the embedment axis, which may coincide with a central longitudinal axis of the body. The lower fins may be aligned angularly with the upper fins about the embedment axis. In that case, each lower fin may be substantially coplanar with a corresponding upper fin of the upper array. Alternatively, the lower fins may be offset angularly with respect to the upper fins about the embedment axis. There can be a longitudinal gap between the upper and lower arrays, or the upper and lower arrays can overlap longitudinally. In the latter case, the upper and lower fins may be in interdigitated relation. The foundation may be embedded in the seabed soil to a mudline that is at a level below an upper side of the upper array. The mudline can be at a level below a lower side of the upper array, below an upper side of the lower array, above an upper side of the lower array, above a lower side of the upper array, above a lower side of the lower array, or between a lower side of the upper array and an upper side of the lower array. For example, the mudline could be in the aforementioned longitudinal gap or longitudinal overlap between the upper and lower arrays. At least one of the lower fins may taper downwardly in cross section, whereas at least one of the upper fins may have an edge that faces away from the body and that is inclined with respect to the embedment axis. The foundation of the invention can be a monopile, in which case the body can comprise a tubular side wall of the monopile and the upper and lower fins can extend from that side wall. The foundation of the invention can instead be a suction pile, in which case the body can comprise a tubular skirt of the suction pile, the lower fins can extend from that skirt and the upper fins can extend across a top plate of the suction pile. The upper fins can overhang or project horizontally beyond the top plate, and outer portions of the upper fins overhanging beyond the top plate can extend downwardly along the skirt. The invention aims for greater efficiency by arranging fins for pile capacity and fins for scour protection in different ways. Also, by stabilising the pile in the seabed and improving bearing capacity, embedded fins can mitigate consequences of such scour as may occur. The invention provides integrated scour protection for a subsea foundation such as a fixed wind monopile or suction pile, in particular by integrating two extra functions or features into a conventional foundation. Those features are integrated into the foundation at different longitudinal positions or heights and can be composed of different angles, numbers or shapes of elements to interact with beneficial effect. In a lower section of the foundation, fins or wings are provided to improve the bearing capacity of the foundation, hence to reduce the size of the foundation for a given load capacity. In addition, in an upper section of the foundation, at or above a seabed or mudline level, a star-like arrangement of plates provides scour protection for the foundation as an alternative to or as a supplement to rock dumping. The fins, wings or plates are apt to be made of steel. Integrating the aforementioned features onto a foundation enables them to be installed with the foundation at an installation location on the seabed. Whilst this may introduce some lifting constraints, it eliminates installation constraints that could otherwise arise if installing such features before or after installing the foundation. Embodiments of the invention provide a subsea foundation such as a monopile or a suction pile comprising: at least a first set of external fins at least partially embedded in the seabed; and at least a second set of external fins at least partially above the seabed. The fins may be disposed radially with respect to the foundation. The first and second sets of fins may be offset angularly relative to each other. The fins of the first set could be totally embedded in the seabed whereas the fins of the second set could be totally above the mudline. Conversely, the fins of the first set may emerge from the seabed so that water flow is guided differently by the first and second set of fins. In summary, a subsea foundation of the invention exemplified by a monopile or a suction pile comprises an embeddable body extending along an upright embedment axis that may coincide with a central longitudinal axis of the body. A plurality of fins extend outwardly from the body in directions transverse to the embedment axis. At least some fins of the plurality are at least partially embedded in seabed soil. The plurality of fins is divided into an upper array of upper fins and a lower array of lower fins. The arrays may be spaced apart or overlap along the embedment axis, and may be angularly aligned or angularly offset with respect to each other about that axis. In order that the invention can be more readily understood, reference will now be made, by way of example, to the accompanying drawings in which: Figures 1 to 3 are schematic perspective views of variants of the invention applied to a monopile foundation embedded in the seabed; Figures 4 to 8 are schematic side views of further variants of the invention applied to a monopile foundation embedded in the seabed; and Figures 9 and 10 are schematic perspective views of variants of the invention applied to a suction pile foundation embedded in the seabed. Figures 1 to 8 show a portion of a monopile 10 of the invention partially embedded in soil of the seabed 12, whereas Figures 9 and 10 show a suction pile 14 of the invention, similarly embedded in the seabed 12. In typical examples, the monopile 10 is apt to support the mast of an offshore bottom-fixed wind turbine whereas the suction pile 14 is apt to support one of the legs of a jacket structure that is itself surmounted by an offshore structure such as a platform or a wind turbine. The monopile 10 or the suction pile 14 could of course serve as foundations for other structures as known in the art. In each drawing, the upper boundary of the seabed 12, namely the nominally planar mudline 16, is shown as a dashed horizontal line drawn on the monopile 10 or the suction pile 14. A lower portion of the monopile 10 or the suction pile 14 extends downwardly beneath the mudline 16 whereas an upper portion of the monopile 10 or the suction pile 14 extends upwardly above the mudline 16. Features of the monopile 10 or the suction pile 14 embedded in the seabed 12 beneath the mudline 16 are also depicted in dashed lines whereas features extending into the water column above the mudline 16 are depicted in solid lines. In the description that follows, references to upper, lower and related directions or positions refer to the monopile 10 or the suction pile 14 in its orientation of use, in which a central longitudinal axis 18 of the foundation is substantially vertical as shown. The skilled addressee will readily understand how features described with reference to the orientation of use will be reoriented or repositioned relative to each other if the monopile 10 or the suction pile 14 is oriented differently, for example when being fabricated at a yard or being transported to an installation site. The monopile 10 is exemplified in Figures 1 to 8 by an elongate monotube of circular crosssection, whose side wall 20 is rotationally symmetrical about a central longitudinal axis 18. The central longitudinal axis 18 coincides with an axis of embedment along which the monopile 10 is advanced into the seabed 12. The short portion of the monopile 10 shown in those drawings has a substantially constant diameter along its length but the monopile 10 as a whole could taper upwardly or have a top portion that is otherwise narrower than a bottom portion. Sets or circumferential arrays 22 of fins 24, namely an upper array 22U of upper fins 24U and a lower array 22L of lower fins 24L, project outwardly from the side wall 20 of the monopile 10. The fins 24 may conveniently be fabricated of steel, welded to the side wall 20 of the monopile 10 which may similarly be made of steel. In these examples, the number of upper fins 24U in the upper array 22U matches the number of lower fins 24L in the lower array 22L, but that is not essential. The upper fins 24U are disposed at a longitudinal position, or level, that is generally above a level of the lower fins 24L. However, as will be explained, lower ends 26 of the upper fins 24U could be at a level below upper ends 28 of the lower fins 24L in some embodiments. In the examples shown, the fins 24 of each array 22 are equi-angularly spaced around the central longitudinal axis 18 and lie in respective radial planes that converge on and contain that axis 18. Moreover, the fins 24 are elongate in a direction parallel to the central longitudinal axis 18. Also, the fins 24 of each array 22 are exemplified here as all being at the same level as each other, and all of the same shape as each other. In both Figures 1 and 2, the fins 24 are radially-oriented, longitudinally-elongate and planar rectangular plates. The lower fins 24L are embedded fully in the seabed 12 below the mudline 16 whereas the upper fins 24U are supported just above the mudline 16, clear of the seabed 12. Thus, there is a longitudinally-extending and circumferentially-continuous gap 30 between the lower ends 26 of the upper fins 24U and the upper ends 28 of the lower fins 24L. In these examples, part of the gap 30 is above the mudline 16 and so water can flow through the gap 30. In Figure 1, the upper fins 24U are in mutual angular alignment with the lower fins 24L. Thus, in this example, each upper fin 24U is coplanar with a corresponding lower fin 24L. Conversely, in the variant of Figure 2, the upper fins 24U are offset or staggered angularly with respect to the lower fins 24L. Thus, in this example, each upper fin 24U is disposed at an angle that lies between the angles of neighbouring lower fins 24L of the lower array 22L, with respect to central longitudinal axis 18. Indeed, each upper fin 24U is positioned and oriented to bisect the angle between neighbouring lower fins 24L of the lower array 22L. Angular offset between fins 24 of the upper and lower arrays 22U, 22L, as shown in Figure 2, is also evident in the arrangements of Figures 3 to 5. Here, however, the fins 24 are further elongated and the arrays 22U, 22L are brought together longitudinally to the extent that there is a longitudinal overlap 32 between the arrays 22U, 22L. The fins 24 are thereby interdigitated, with the lower end 26 of each upper fin 24U being received between upper ends 28 of neighbouring lower fins 24L of the lower array 22L. Thus, there is no longer a longitudinally-extending gap 30 between the arrays 22U, 22L. It will be apparent that to flow around the side wall 20 of the monopile 10, an incident fluid flow must follow a path that undulates in alternation under the upper fins 24U and over the lower fins 24L. This effectively breaks up a horseshoe vortex above the mudline 16 while also using the partially embedded fins 24L to improve the bearing capacity of the monopile 10. More generally, these and other embodiments exemplify the inventive principle of dividing the fins 24 into upper and lower arrays 22, which allows the fins 24, and potentially the same fins 24, to perform the dual purpose of improving bearing capacity and mitigating scour. In this respect, bearing capacity is improved to the extent that fins 24 are embedded in the seabed 12 whereas scour is mitigated to the extent that fins 24 extend above or are disposed above the mudline 16. The mudline 16 can be at various levels along the monopile 10 with respect to the overlap 32 between the upper and lower arrays 22U, 22L of fins 24. For example, in Figure 3, the overlap 32 is bisected by the mudline 16. Thus, the lower ends 26 of the upper fins 24U are embedded in the seabed 12 beneath the mudline 16 whereas the upper ends 28 of the lower fins 24L protrude above the seabed 12. In Figure 4, the overlap 32 lies above the mudline 16. Thus, the upper ends 28 of the lower fins 24L again protrude above the seabed 12 but in this case the upper fins 24U lie wholly above the seabed 12 with their lower ends 26 held clear of the mudline 16. Conversely, in Figure 5, the overlap 32 lies beneath the mudline 16. Thus, the lower fins 24L lie wholly embedded in the seabed 12 with their upper ends 28 below the mudline 16 whereas the lower ends 26 of the upper fins 24U are also embedded in the seabed 12 below the mudline 16. Similarly, Figures 6 to 8 show the mudline 16 at various levels along the monopile 10 in arrangements where, as in Figure 1, there is angular alignment between the fins 24 of the upper and lower arrays 22U, 22L. In Figure 6, as in Figure 5, the lower fins 24L lie wholly embedded in the seabed 12 with their upper ends 28 below the mudline 16 and the lower ends 26 of the upper fins 24U are also embedded in the seabed 12 below the mudline 16. In Figure 7, the lower fins 24L again lie wholly embedded in the seabed 12 with their upper ends 28 below the mudline 16 but in this case the lower ends 26 of the upper fins 24U are substantially aligned with the mudline 16, hence touching the seabed 12 but not being significantly embedded. In Figure 8, as in Figure 4, the upper ends 28 of the lower fins 24L protrude above the seabed 12 and the upper fins 24U lie wholly above the seabed 12 with their lower ends 26 held clear of the mudline 16. Figures 6 to 8 also illustrate that the fins 24 of either array 22 need not necessarily be rectangular but could have other shapes. In those examples, the upper fins 24U taper downwardly as they radiate outwardly from the monopile 10, hence defining radially outer edges 34 that are inclined relative to the central longitudinal axis 18 and that together define an upwardly-tapering frusto-conical arrangement. Moreover, Figures 7 and 8 illustrate that the fins 24 of either array 22 need not necessarily be planar but could have other shapes or cross-sections. In those examples, each lower fin 24U tapers downwardly in cross-section to define a downwardly-facing blade formation 36 at its lower edge that eases penetration into the soil of the seabed 12. Returning finally to Figures 9 and 10, the suction pile 14 shown in these drawings has a tubular skirt 38 that is rotationally symmetrical about a central longitudinal axis 18. As before, the central longitudinal axis 18 coincides with an axis of embedment along which the suction pile 14 is advanced into the seabed 12. As is conventional, the top of the skirt 38 is closed by a top plate 40 that defines a suction chamber within the skirt 38 between the top plate 40 and the soil of the seabed 12 encircled by the skirt 38. In this example, the top plate 40 is surmounted by a tubular socket 42 that opens upwardly to serve as a connector for a leg of a jacket. As in the preceding embodiments, the suction pile 14 supports two circumferential arrays 22 of fins 24, namely an upper array 22U of upper fins 24U and a lower array 22L of lower fins 24L. As before, the fins 24 may conveniently be fabricated of steel, being welded to the steel skirt 38, the top plate 40 and / or the socket 42 of the suction pile 14 as appropriate. In the examples shown, as before, the fins 24 of each array 22 are equi-angularly spaced around the central longitudinal axis 18 and lie in respective radial planes that converge on and contain that axis 18. Again, the number of upper fins 24U matches the number of lower fins 24L but that remains inessential. The lower fins 24L project outwardly from the skirt 38 whereas the upper fins 24U extend across the top plate 40 of the suction pile 14. In Figures 9 and 10, as previously, the lower fins 24L are exemplified by radially-oriented, longitudinally-elongate planar rectangular plates but they could have other shapes or sections. Conversely, in this example, the upper fins 24U taper downwardly as they radiate outwardly from the socket 40, hence defining upper edges 44 that are inclined relative to the central longitudinal axis 18 and that together define an upwardly-tapering frusto-conical arrangement. More generally, any of the fins 24 or either of the arrays 22U, 22L shown in Figures 9 and 10 could have any of the features described in relation to the embodiments of any of Figures 1 to 8. As in Figures 1 and 2, the lower fins 24L shown in Figures 9 and 10 are embedded fully in the seabed 12 below the mudline 16 whereas the upper fins 24U are held just above the mudline 16, clear of the seabed 12. Also, as in Figure 2, the upper fins 24U are offset angularly with respect to the lower fins 24L. However, the fins 24 of the respective arrays 22U, 22L could be at other levels with respect to each other and with respect to the mudline 16 as shown in preceding embodiments. For example, the upper fins 24U could be in mutual angular alignment with the lower fins 24L, with each upper fin 24U being coplanar with a corresponding lower fin 24U in a manner akin to that shown in Figure 1. Conversely, angularly-offset arrays 22U, 22L could overlap longitudinally with their fins 24 in interdigitated relation as described previously with reference to Figure 3. In this respect, the upper fins 24U extend radially beyond the outer circumference of the top plate 40, hence overhanging the exterior of the skirt 38. The lower edges 32 of the upper fins 24U lie level with the top plate 40 in Figure 9. So, as the top plate 40 typically lies above the mudline 16, the upper fins 24U also lie above the mudline 16. However, the overhanging outer end portions 46 of the upper fins 24U shown in Figure 10 also extend down the exterior of the skirt 38 beneath the level of the top plate 40 and, in this example, extend beneath the mudline 16. In a variant of the arrangement shown in Figure 10, it would be possible for the lower ends 26 of the upper fins 24U, defined by the outer end portions 46, to lie below the level of the upper ends 28 of the lower fins 24L so that the upper array 22U overlaps longitudinally with the lower array 22L. It would also be possible for the upper fins 24U to radiate outwardly from the skirt 38 of the pile, like the lower fins 24L, and therefore to be embedded at least partially in the seabed 12 like the upper fins 24U of preceding embodiments. Many other variations are possible within the inventive concept. The fins need not be equi-angularly spaced around the central longitudinal axis and need not lie in planes that contain that axis, or indeed be planar at all. For example, the fins could be inclined relative to the central longitudinal axis or could have various non-planar or non-elongate shapes. The fins of each array need not all be at the same level as each other but could instead be at staggered or circumferentially alternating levels. The number of fins in each array need not be identical: one array could have more or fewer fins than the other array. Also, there could be more than two arrays spaced longitudinally along the foundation.

Claims

1. A subsea foundation comprising: a body extending along an upright embedment axis; and a plurality of fins that extend from the body in directions transverse to the embedment axis, wherein the plurality of fins is divided into an upper array of upper fins and a lower array of lower fins, and at least some fins of the plurality are at least partially embedded in seabed soil, and wherein:the lower fins are offset angularly with respect to the upper fins about the embedment axis; and / orthe foundation comprises a longitudinal gap between the upper and lower arrays.

2. The foundation of Claim 1, wherein the lower fins are aligned angularly with the upper fins about the embedment axis.

3. The foundation of Claim 2, wherein each lower fin is substantially coplanar with a corresponding upper fin of the upper array.

4. The foundation of Claim 1, wherein the upper and lower arrays overlap longitudinally.

5. The foundation of Claim 4, wherein the lower fins are in interdigitated relation with the upper fins.

6. The foundation of any preceding claim, when embedded in the seabed soil to a mudline at a level below an upper side of the upper array.

7. The foundation of Claim 6, wherein the mudline is at a level below a lower side of the upper array.

8. The foundation of Claim 6 or Claim 7, wherein the mudline is at a level below an upper side of the lower array.

9. The foundation of Claim 6, wherein the mudline is at a level above an upper side of the lower array.

10. The foundation of Claim 9, wherein the mudline is at a level above a lower side of the upper array.

11. The foundation of any of Claims 6 to 10, wherein the mudline is at a level above a lower side of the lower array.

12. The foundation of Claim 6, wherein the mudline is at a level between a lower side of the upper array and an upper side of the lower array.

13. The foundation of Claim 12, wherein the mudline is in the longitudinal gap between the upper and lower arrays.

14. The foundation of Claim 12 when appendant to Claim 4, wherein the mudline is in a longitudinal overlap between the upper and lower arrays.

15. The foundation of any preceding claim, being a monopile wherein the body comprises a tubular side wall of the monopile and the upper and lower fins extend from that side wall.

16. The foundation of any of Claims 1 to 14, being a suction pile wherein the body comprises a tubular skirt of the suction pile, the lower fins extend from that skirt and the upper fins extend across a top plate of the suction pile.

17. The foundation of Claim 16, wherein the upper fins project horizontally beyond the top plate.

18. The foundation of Claim 17, wherein outer portions of the upper fins projecting beyond the top plate extend downwardly along the skirt.

19. The foundation of any preceding claim, wherein the fins lie in respective radial planes that converge on and contain the embedment axis.

20. The foundation of any preceding claim, wherein the embedment axis coincides with a central longitudinal axis of the body.

21. The foundation of any preceding claim, wherein at least one of the lower fins tapers downwardly in cross section.

22. The foundation of any preceding claim, wherein at least one of the upper fins has an edge that faces away from the body and that is inclined with respect to the embedment axis.

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